T14 Thill...2X within one foot of the other

[Holeshot]

Here's a nice little mystery (which I think we figured out):

Rounds 3 & 4 I put the bike on the ground during the AFM weekend in turns 14 and T11 at Thill. This the first time on the ground in several years and probably over 50 individual races. These were weird ones:

Crash 1: Bike felt quick and had plenty of grip. Bike felt confident in the front end and didn't have any ill handing issues on entry, mid corner, or exit. Practice and F1 qualifying were reasonable. I had 1 race on Saturday that went well and two more on Sunday which both went very well. 600 Superstock was my third race and later in the day. I was able to make more and more time into T14 at Thill during the race to make up time on the leader. After 3 laps of making up time, I turned into T14 on the same line I'd used the whole weekend, gave away lever as I added lean angle and was a bit more than 5% or so at the apex. @ the apex, the front end collapsed with no warning. No heavy tire feel, no wagging bars...just bang: on the ground. I chalked this up to holding too much lever to the apex.

Crash 2: Bike felt decent in practice (saturday). Front was a bit vague feeling when I'd transition to the side of the tire and start giving away brake lever, but not a deal breaker. During qualifying, I ran my first hot lap for an initial lap time. The second lap, I was using the rider (my prime competition) who was about 3 seconds ahead as a marker to catch, for a faster lap time. The lap felt pretty darn fast given the time made up on him. Enter T14, I did everything the same; braked in the same spot, turned in at the same spot, gave lever away all throughout the turn at the same spot. @ the apex of T14, with maybe 5% brake lever and almost immediately before transitioning to throttle, the front end collapsed with no warning. No bobble, no slide, no heavy bar then collapse, no light bar wagging; just, Collapsed. Oddly enough, the bike hit the ground within a one foot of the my previous AFM weekend at Thill's 600 Superstock crash. I've never crashed in almost the same spot in the same exact manner before. We got exactly 1 lap in for qualifying before putting the bike on the ground. It was very hot this weekend (110 degrees or so...probably about 105 at the time of losing the front). At this point, I didn't know what I'd done wrong, but was starting to suspect something different from rider error given the mirrored event to my previous crash in 600 super stock one month previous.

Crash 3: Same race weekend, Sunday. I had realized something was up with maybe bike balance or something, but began pussyfooting corner entries and getting off the lever much earlier than normal and not carrying as much lever and lean angle as I have several years of my racing now. It was slower and more difficult to get the bike work well, but not putting as much load on the front end. In the 600 super stock race, I got passed by Velentin Debise for the lead going into T9, raced into T10 with him and then followed him for a lap. Our next entry into T11, I tried his move of moving his head/ shoulders more to the inside a tad more. Once again, I was off the brakes and almost at the exact moment of transition to throttle, the front collapsed. I entered the corner a slight amount slower than Valentin, and did the same as always except for trying to hold the bike down the apex a bit longer with body weight. The collapse had no warning again. The front also was vague this weekend.


Changes we made to the bike from the last time we were at Thill: gearing change from 14/43 to 15/46. Ratio is the same, sprocket size is different. This necessitated a chain length change.

After scratching our heads, at dinner we came up with the following additional info: wheelbase was moved rearwards about 35-45 mm or so and not corrected on mistake. I run a very short wheelbase and that wasn't corrected. The move of the rear wheel rearward put more weight on the front end. We concluded that we did not make the correct adjustment to raise (add preload) the front forks/ change the weight bias rearward. This in turn had the forks on the bottom at the point of the smallest contact patch for the front tire. Possibly a bit too fast of front rebound may have helped the front end extend once grip was momentarily lost with no chance to regrip up.

This was our conclusion. This being Crash Analysis, discussion and comments are always welcomed. This weekend we had made the appropriate changes and had no issues.

 

[Enchanter]

Good stuff!

The data geek in me wants to know the weight on each contact patch pre and post wheelbase change.

There has to be a formula out there already.

 

[Holeshot]

Interesting you ask that. Several years ago, Siglin and I bought postal scales and measured the weight from moving from the front of the seat to the back of the seat: result was 12 lbs to the rear. Not as much as I'd guess.

One of my friends I race with (Robert Brittain; motogloss paint) put brake/ throttle cameras on my bike for the last two races of the day. I'm interested to see what comes out of that...how much neutral throttle I hold. His insistence came from my critique on where he was losing time all over track from his videos he posted; neutral throttle all over, in between brake/ throttle for way too long.

Fingers crossed I'm not doing that badly!

 

[afm199]

Only comment I have is that it was a pretty cool, relatively, race weekend, and lots of riders were having grip issues. And, having reread the post, this makes no difference and is not applicable/

 

[puckles]

I am far from an expert, but I had an identical apex crash at laguna turn 5 that I'm still trying to figure out the cause. Switching from q3's to slicks seemed to make my bike handle much more confidently, even though im pretty slow and am not even close to pushing the tire. I noticed that the front end stopped (what felt like) skating across the bumps at the apex of 11 at thill after swapping tires. I wonder if the tire diameters were different and gave me different weight distribution.

I'm interested to see what people think about your crashes, hopefully you figure it out.

 

[stangmx13]

It’s unlikely u were near bottom at the apex with 0-5% brake pressure. Turning doesn’t create nearly as much load as braking. The corner is also flat so there’s no g-out. Maybe we can find someone with suspension pots and a Thill lap to see a possible suspension position for that corner.

Typo in your wheelbase changes? 4 total teeth ~= 16mm, 2 links ~= 16mm. And they are opposite changes. I don’t see how u got 35-45mm.

3.5-4.5mm is more likely. Such a wheelbase change won’t adjust the weight balance enough to put the forks on bottom at apex. Maybe you’d want 0-2mm fork preload to compensate, which is small.

I’d bet you wouldn’t have crashed if you held more brake pressure. My experience is that overloaded tires provide feedback and underloaded ones don’t. But that’s on Dunlops. More preload might help because it could give u confidence to hold the lever harder and deeper. But I really doubt you overloaded the front tire.

 

[Holeshot]

I think there are a few things that either got misinterpreted;

1. At max lean, I want off the lever when it's time to go to the throttle. T14 is a flat(ish) corner and I make most of time on the side of the tire for my entries. Holding more lever in T14 would be the same problem only earlier in the corner, IMO.

2. 2 links = 35mm. One is about 16mm. using only the axle position as the measurement, the wheel was moved rearward about 35 mm or so from the previous measure we had on it. 43-46 too is 3 teeth, for whatever that is worth.

I still have nothing more. Rode the last race weekend with the original WB and bike was good, but twitchy as hell, which I don't remember from previous time we had it setup that way. Put it more on it's nose and I started to get front end feel back. I still think I might go back to a longer wheelbase for mid corner stability for the last AFM round and see how that works. Or do one link added and run with that.

Thanks for the thoughts Rob.

 

[Holeshot]

Update: measured shock and discovered the bike sits high in the back as is. We settled on the geometry being affected as the issue. Didn't get a chance to walk T14 apex, but another of my close competitors went down in the same spot over the weekend. Suspect the combination of geometry, more weight on the front end and excessive lean angle as the prime contributors...least that's what I'm telling myself.

 

[afm199]

Same place I crashed tuesday, right before the apex, carrying too much lean angle and off the breaks. Stupid.

 

[Holeshot]

You too Ernie? Just let go w/ no warning?

 

[afm199]

You too Ernie? Just let go w/ no warning?

There and gone no warning, no push. Second crash in the same place. Like the first one, everything was fine until I was on the ground.

I did get warning. About a half a lap prior my head said:" Dude you need to back out of this pace and call it a day, you're straining." I should have listened.

For me the transition between brake and throttle there, with lots of lean angle, is a bit treacherous.

 

[Holeshot]

Same exact thing Ernie....exactly the same.

 

[tzrider]

I haven't been there in years: Are there bumps, seams or patches in that area? With any amount of brake still applied, there will also be some residual bar pressure. If there are surface irregularities, the front will have a harder time following contours with pressure on the bar and high lean angle.

With these incidents all seeming to happen in the same place, there has got to be something about the surface that is contributing.

 

[Holeshot]

Good analysis Andy. I'm thinking something with the surface for sure. I meant to walk it this last weekend and didn't get the chance. If I go up for Z2 in a few weeks, I'm gonna walk it for sure. I'll check back then.

 

[afm199]

It's an interesting place. We want to get as much speed as possible for T15, which is SO IMPORTANT, but at the same time don't want to overwhelm the front with lean angle. We all want as much drive as possible on the exit of T15, on the little bikes we need all the roll speed as well. On the bigger bikes I am comfortable trail braking right to the apex and then rolling on, my crashes both were on the SV, where I was desperately (the correct word) looking for lean angle and roll speed. I gave up the brake a bit early and that's what got me. Of all the corners at Thill, this is the only one that I just don't like on the SV. The front feels wooden and vague and I have no idea what's going on.

I'd agree about the surface but even at my perceived apex and Berto's, there's probably five feet of track both ways between us.

 

[stangmx13]

I think there are a few things that either got misinterpreted;

1. At max lean, I want off the lever when it's time to go to the throttle. T14 is a flat(ish) corner and I make most of time on the side of the tire for my entries. Holding more lever in T14 would be the same problem only earlier in the corner, IMO.

2. 2 links = 35mm. One is about 16mm. using only the axle position as the measurement, the wheel was moved rearward about 35 mm or so from the previous measure we had on it. 43-46 too is 3 teeth, for whatever that is worth.

I still have nothing more. Rode the last race weekend with the original WB and bike was good, but twitchy as hell, which I don't remember from previous time we had it setup that way. Put it more on it's nose and I started to get front end feel back. I still think I might go back to a longer wheelbase for mid corner stability for the last AFM round and see how that works. Or do one link added and run with that.

Thanks for the thoughts Rob.

holding more lever will prevent u from trying to carry too much pace, too much lean angle. front crashes in between brake and gas at our pace are usually caused by some combination of excessive lean angle and too little grip. ive crashed like that too many times :laughing. u can fix both of those by holding some brake longer. it doesnt just "move" the problem because your speed is constantly decreasing.

lets switch to using chain length instead. the only way you can get ~32mm of axle movement is 14/43-110 to 15/46-116. thats a 6 link change which is nuts. I dont even think a 110 chain fits with those sprocket. the slot only has ~45mm of adjustment. if it does fit, you'd def be within a few mm of the very front. im shocked anyone would ever run the axle there. any less of a chain length change is less axle movement of course.

15/46-116 is within a few mm of both 15/42-114 and 15/43-114. I run those 2 combos a lot and like them. every setup ive seen in MA is there too. so you arent out of range. but 16mm is def a big change from what you were running previously.

Update: measured shock and discovered the bike sits high in the back as is. We settled on the geometry being affected as the issue. Didn't get a chance to walk T14 apex, but another of my close competitors went down in the same spot over the weekend. Suspect the combination of geometry, more weight on the front end and excessive lean angle as the prime contributors...least that's what I'm telling myself.

one side effect of more swingarm length is less rake and trail in the front end. because the swingarm spends almost all of its time angled down, moving the axle back also moves it downward. so the bike is pitched forward a little bit. if you already have the shock long, your real trail number might be small. bikes with less rake and less trail are known to tuck unexpectedly. they have plenty of grip, but when the tire lets go, it goes fast.

this is just a guess without knowing the height of your front end.

 

[stangmx13]

It's an interesting place. We want to get as much speed as possible for T15, which is SO IMPORTANT, but at the same time don't want to overwhelm the front with lean angle. We all want as much drive as possible on the exit of T15, on the little bikes we need all the roll speed as well. On the bigger bikes I am comfortable trail braking right to the apex and then rolling on, my crashes both were on the SV, where I was desperately (the correct word) looking for lean angle and roll speed. I gave up the brake a bit early and that's what got me. Of all the corners at Thill, this is the only one that I just don't like on the SV. The front feels wooden and vague and I have no idea what's going on.

I'd agree about the surface but even at my perceived apex and Berto's, there's probably five feet of track both ways between us.

ive done that too many times. the most recent one was Sept '18 in Alabama. I let off the brakes early in an attempt to keep my roll speed up through one of the faster corners and the front just let go. later in the weekend, i used the "opposite" technique to go faster - I braked later. that helped a lot :laughing

 

[Holeshot]

if it does fit, you'd def be within a few mm of the very front. im shocked anyone would ever run the axle there. any less of a chain length change is less axle movement of course.

one side effect of more swingarm length is less rake and trail in the front end. because the swingarm spends almost all of its time angled down, moving the axle back also moves it downward. so the bike is pitched forward a little bit. if you already have the shock long, your real trail number might be small. bikes with less rake and less trail are known to tuck unexpectedly. they have plenty of grip, but when the tire lets go, it goes fast.

this is just a guess without knowing the height of your front end.

Good point on the thing tucking with less trail. I run the rear wheel packed up in the swingarm; the tire warmers always rub putting them on. No one seems to run it that way (so I'm told), but then again, it's worked for me for now and gives me a bike that flicks quickly and rips through T3 at Thill. I'm kinda ready to try something different though cause the thing can probably be better...but not at turning under people, etc. I should take a pic of where the rear wheel is...you'd probably chastise me for sure.

 

[afm199]

ive done that too many times. the most recent one was Sept '18 in Alabama. I let off the brakes early in an attempt to keep my roll speed up through one of the faster corners and the front just let go. later in the weekend, i used the "opposite" technique to go faster - I braked later. that helped a lot :laughing

Amen. As soon as the front tucked I was kicking myself in the ass, knowing that sliding across the track is definitely not faster than riding around the corner. I suspect if I had kept on the last two ounces of brake for another five feet, I would have had no problems at all. The poor front tire just didn't have the contact patch to handle my demands.

And that was a fucked up crash! I got hit by a buddy and broke my C5, still sore as hell and waiting for full nerve function to return. Getting better every day, but still about 70% of full function.

 

[stangmx13]

Good point on the thing tucking with less trail. I run the rear wheel packed up in the swingarm; the tire warmers always rub putting them on. No one seems to run it that way (so I'm told), but then again, it's worked for me for now and gives me a bike that flicks quickly and rips through T3 at Thill. I'm kinda ready to try something different though cause the thing can probably be better...but not at turning under people, etc. I should take a pic of where the rear wheel is...you'd probably chastise me for sure.

I have plenty of bias when doing my own setup. Ive always had my front end tall and was attached to it. The last time I raised it, it worked better, so why would I shorten it now? Ive always liked soft shock springs and was attached to those because going softer worked better too. But the shock hasn't gripped well in over a year and the front end now seems to work better with lighter springs. Of course, I should have tried stiffer rear springs and lowering the front a year ago. but I didnt because of my bias.

last time out in Alabama, I started working with Ken Hall (SBSuspension, Ohlins, Bryce Prince's technician) and he suggested all the changes that I wouldn't make myself. They worked and I went a lot faster. He actually told me exactly what I needed to hear too - something like "that old setup does work well to a point. but if you want to go faster, we gotta go in a diff direction".

we all know that setups are good for specific tracks, tires, and riders. as we get faster, we become different riders. so its silly to assume some setup will always work as we improve. your short wheelbase might be the same as my super-tall front end. might not though.